Hallucination as a false perception has acquired culturally different meanings, among others for shamanism and religions. Hallucinations have been defined in the course of the development of psychiatry, with the distinction between hallucinations, illusions, and ego disturbances being clearly determined: Hallucinations were understood as perceptions without a corresponding stimulus, while illusions were defined as misinterpretations of perceived objects—for example, perceiving a grotesque face in a harmless wall drawing. Ego disturbances (see next chapter), unlike hallucinations, generally do not have sensory qualities. Hallucinations vary in many dimensions, which are explained in this chapter.